In optical communications networks, an Optical Service Channel (OSC) is used to convey operation administration and maintenance (OAM) signalling for managing line equipment. In Wavelength Division Multiplexed (WDM) optical communications systems, the optical service channel is typically an intensity modulated channel that is co-propagated with the data channels. This enables the same transmitter to be used to send the OAM signalling as well as subscriber traffic.
FIG. 1a schematically illustrates a representative optical fibre link 2 in a conventional WDM optical communications system. In the illustrated example, the fibre link 2 comprises three optical fiber spans 4 extending between a transmitter 6 and a receiver 8, and traverses an optical amplifier 10 and an Optical Add-Drop Multiplexers (OADM) 12. As is well known in the art, optical fibre links commonly have multiple spans, and include a variety of optical devices, such as optical amplifiers and OADMs, for example. Transmitters and receivers are commonly incorporated into network nodes which provide some combination of signal regeneration, electrical switching (such as wavelength switching), and layer-2 (or higher) signal routing functionality. Typically, a bidirectional optical link comprises a pair of parallel fibre links 2 extending between the two end nodes. Normally, these parallel fibre links will be constructed as a “mirror image” of each other, so as to convey optical signals in respective opposite directions. For this reason, only one fibre link is shown in FIG. 1a. 
The transmitter 6 generates a WDM optical signal comprising a plurality of data channels λDATA and at least one optical service channel λOSC, as may be seen in FIG. 1b. Typically, the optical spectrum of the WDM signal will follow a standard spectral grid such as one of the spectral grids specified by the International Telecommunications Union (ITU), for example.
Typically, data channels λDATA of the link are considered to extend through the entire link 2 from the transmitter 6 to the receiver 8, and thus maintain continuity through each of the intermediate optical devices 10, 12. On the other hand, an optical service channel λOSC is limited to one span 4, and thus is terminated at each of the intermediate optical devices 10, 12.
Accordingly, at each optical device within the link 2 (in this example, the optical amplifier 10 and the OADM 12), an optical coupler 16 (such as, for example, a passive filter-based optical demux) is used to separate the optical service channel λOSC from the fibre link, and supply the optical service channel λOSC to an OAM controller unit 18. In the illustrated example, the OAM controller unit 18 includes an OSC receiver 20; a processor 22; a regenerator 24 and an OSC transmitter 26. The OSC receiver 20 terminates the OSC channel λOSC and recovers the OAM messages modulated on the OSC channel λOSC. The processor 22 may operate under software control to, among other things, implement OAM functions in respect of the respective optical device 10, 12. The processor 22 may also generate OAM messages (eg status reports and alarm notifications), which are passed to the OSC transmitter 26 for transmission through the OSC channel λOSC of the next span. The regenerator 24 can be used to implement a “pass-through” function, so that received OAM messages that are not destined for the OAM controller 18 can be passed to the OSC transmitter 26 for transmission through the OSC channel λOSC of the next span. A second optical coupler 28 (such as, for example, a passive filter-based optical mux) adds the optical service channel λOSC from the OSC transmitter 26 to the WDM signal for transmission through the next span.
At the receiver 8, the Optical Service Channel λOSC is demultiplexed from the WDM signal and received in a conventional manner. The received OAM signals then can be forwarded by the receiver 8 to a central network server (not shown) in a manner well known in the art.
With this arrangement, each OAM controller unit 18 can receive OAM messages through the inbound optical service channel λOSC, and thereby implement management functionality in respect of the associated optical device. In addition, the OAM controller unit 18 can generate and transmit OAM messages through the next span optical service channel λOSC. The regenerator 22 enables the OAM controller unit 18 to regenerate and transmit OAM signals pertaining to other OAM controller units 18 on the same fibre link 2. In the case of multi-span fibre links, this operation enables any given OAM controller unit 18 to communicate with a central network management server (not shown), with OAM messages to and from the given OAM controller unit 18 being relayed through the transmitter 6 and receiver 8, and any intermediate OAM controller units 18 on the fibre link 2
As is well known in the art, the arrangement described above enables effective implementation of OAM functionality in each respect of each optical device 10, 12 along the optical fibre link 2. However, this arrangement also suffers a limitation in that fiber non-linear effects such as Cross-Phase Modulation (XPM) and four-wave mixing can cause interference between the optical service channel λOSC and the data channels λDATA. When data channels operate using intensity modulation direct detection, the magnitude of the signal degradation due to this is interference is typically much lower than dispersion, and thus can be tolerated. However, when data transmission relies on detection of phase modulation non-linear distortions in the data channels λDATA due to OAM signalling in the optical service channel λOSC can cause significant degradation in the SNR of the data channels λDATA.
Techniques that mitigate interference between the optical service channel λOSC and the data channels λDATA in a Wavelength Division Multiplexed (WDM) optical communications system remain highly desirable.